I am installing a new fiberglass shower surround and basin in an old bathroom. I had a carpenter install drywall which is suppose to meet the top of the new shower basin. However on the rear wall; the drywall comes over the lip of the basin. There is now a half inch space between the drywall and shower basin lip. Neither I nor my plumber can figure how to compensate for this gap. We need to apply a silicon sealant to the edge of basin and drywall all the way around. But the gap will cause the sealant to fall through. We would appreciate any suggestions. Thank you.